[Laser] Sensitivity to Sound Card Noise

[Art]

OK, thanks to Steve, Yves, and the group for their patience and for 
getting me straightened out.

I think I've got it now::>

When looking at the power levels as Steve suggested below, it becomes 
much clearer-at least in my mind.

I'll rest easier knowing that sound card selection isn't as critical 
as I had thought it to be previously.

As to the performance of the PGP detector relative to the op amp 
transimpedance front end, I think the book is still open though.

I think the only way to really know the answer is to test them side 
by side in a physical environment that accounts for the difference in 
the active area of the photodiode in both types of preamps. Or, 
stated another way....if both preamps under test have the same size 
photodiodes, they can be tested side by side. If not, we have to 
adjust the intensity of the light input so that the same amount of 
photons falls on each detector.

Also, the bandwidth of both detectors needs to be similar for the 
test to be fair. Fortunately, the bandwidth of the typical 
photodiode/transimpedance op amp based front end is easily modified.

Signed,

Curious in Maine




>Art therefore has a better total noise level than I
>do, by a power ratio of 10.25/11, which corresponds to
>a 0.31 dB improvement in noise.  Since we are using
>systems which are otherwise identical any desired
>signal level will be the same for both of us, so the
>signal-to-noise ratio is improved by the same amount,
>just 0.31 dB.  With a higher gain preamp the
>improvement due to reduction of sound card noise will
>be even less.